1. Field of the Invention
The present invention relates in general to a network communication device, and specifically to a hybrid circuit used in a frequency division duplexing(FDD) system to protect transmission function and inhibit interference of low-frequency signals generated during the communication process.
2. Description of the Related Art
The demand for transmission speed, time, quality and function of networks has become higher as networks become more and more popular. The network communication method most widely used in high-speed data transmission is XDSL, such as asymmetric digital subscriber line(ADSL) modem, through traditional telephone lines.
Referring to FIG. 1, an ethernet broadband communication system based on XDSL technology is shown. The communication system includes a subscriber 1 and a central office 2. The subscriber 1 is connected to the central office 2 through a twisted pair line. The subscriber 1 includes an XDSL modem 11 and a personal computer or computer network 12. The central office 2 includes a patch panel splitter 21 connected to the XDSL modem 11, a private branch exchange(PBX) 22 connected to the public switched telephone network(PSTN) and an internet access router 23 connected to the internet.
Referring to FIG. 2, a block diagram of the XDSL modem in FIG. 1 is shown. The subscriber is provided with a plain old telephone service(POTS) splitter 13 to connect the XDSL modem 11 and a network line. The XDSL modem 11 has a hybrid circuit 111 to couple a transmitter TX and a receiver RX to the network line. An analog signal is transmitted to the filter 113 of the transmitter TX and the filtered analog signal is transmitted to the hybrid circuit 111. The receiver RX receives the signal from the hybrid circuit 111.
Currently, the communication transformer design used in current FDD networking products uses a hybrid circuit to accomplish insulation/impedance allocation between the primary winding and the secondary winding. The hybrid circuit plays an important role in impedance matching and signal power distribution between the upstream and downstream of a broadband network. However, when the broadband network is in operation, the hybrid circuit can't inhibit/absorb the low-frequency AC signals generated by a ringing or hook on/off such that the packets through the network can not be completely received, i.e. cyclical redundancy check (CRC) error. Furthermore, the network may be disconnected and it generates a significant impact on transmission functionality of internet.